Classification of sandy coastlines
General principles have been presented in Characteristics of sedimentary shores. This article focuses on the local planform of the shore, which is highly dependent on longshore processes. The interaction between longshore processes and the given coastal geology, sediment supply, etc., results in the formation of different types of coastlines and coastal features. Therefore, in order to be able to make an overall evaluation of a certain site in relation to shoreline management activities, it is also relevant to study the coastline features. This is done by dividing the coastlines into nearly straight sections and into special coastal features, such as deltas, barrier islands and spits, etc. The nearly straight coastlines are subdivided into categories dependent on the angle of incidence of the prevailing waves and dependent on wave exposure. The geomorphological processes associated with these physical structures are also fundamental to the way in which the more dynamic habitats and ecosystems of coastal terrestrial areas and transitional waters develop.
Contents
Nearly Straight Coastlines
A coastline classification is given in the following in order to provide an overview of the typical problems, which a certain type of coastline may present for the population living in the specific coastal area, but also to provide guidelines for possible/feasible shoreline management measures.
Only sedimentary coastlines, which are characterised by the presence of loose sediments on the shoreface and on the beach, will be included in the following rough classification.
There are 5 main types of coasts defined by the angle of incidence of the prevailing waves.
- Type 1: Perpendicular wave approach, angle of incidence close to zero
- Type 2: Nearly perpendicular wave approach, angle of incidence 1o - 10o, net transport small to moderate
- Type 3:Moderate oblique wave approach, angle of incidence 10o - 50o, large net transport
- Type 4:Very oblique wave approach, angle of incidence 50o - 85o, large net transport
- Type 5:Nearly coast-parallel wave approach, angle of incidence >85o, net transport near zero
The angle of incidence is measured with respect to the normal to the coastline. This angle of incidence between the coast orientation and the prevailing waves can also be expressed as the angle between the present coastline and the coastline orientation of net zero transport, see the image littoral drift budget.
This classification has been subdivided according to the wave exposure as follows:
- P: Protected, the “once per year event” having Hs,12h/y < 1 m
- M: Moderately exposed, the “once per year event” having 1 m < Hs,12h/y <3m
- E: Exposed, the “once per year event” having Hs,12h/y > 3 m
Characterising coastlines according to the above rules is, of course, not completely straightforward, as there can be seasonal variations, etc. to take into account.
The littoral transport conditions for these coastlines are presented schematically in Fig. 1.
Fig. 2. presents examples of the different coastal types given above. It includes a simplified classification of coastal landforms in relation to geological features and morphological processes. It is evident that there is a close relationship between the resulting morphological features and the type of coast.
The reason why it is important to establish which type of coastline is present at a certain location is that the different types of coastlines react differently to the same shoreline management measure. Recommended shoreline management measures for the various coastline classes are dealt with in Dealing with coastal erosion and Hard coastal protection structures.
The possibility of artificially establishing a practically stable sandy shore depends very much on the incidence angle of the prevailing waves and the magnitude of the transport deficit, which is closely related to the transport capacity.
It should be noted that a prerequisite for obtaining an attractive sandy shore is that the location is exposed or moderately exposed to waves and that it is not exposed to negative correlation between onshore waves and storm surge. It is the constant movement of the beach sand under these circumstances that generates the attractive clean sandy beach.
It should be noted too, that the above given classification is a simplification. Other parameters, such as the sediment supply from the neighbouring areas, as well as seasonal variations in wave climate, tides and storm surges, are also of some importance.
Overview of coastal characteristics
An overview of the coastal characteristics of the different coastal classifications is presented in the table below.
| Coastal Type | Angle of Incidence (0o=shore normal) | Exposure | Main Coastal Characteristics |
|---|---|---|---|
| 1P | 0o | Protected | Marshy |
| 1M | Moderate | Narrow stable sand beach, barrier isl., sand spits | |
| 1E | Exposed | Wide stable sand beach, barrier island, sand spits | |
| 2P | 1o< 10o | Protected | Marshy |
| 2M | Moderate | Narrow stable sand beach, barrier island, sand spits | |
| 2E | Exposed | Wide stable sand beach, barrier island, sand spits | |
| 3P | 10o – 50o | Protected | Marshy |
| 3M | Moderate | Narrow unstable sand/shingle beach, cliff or dunes | |
| 3E | Exposed | Wide unstable sand/shingle beach, cliff or dunes | |
| 4P | 50o – 85o | Protected | Marshy |
| 4M | Moderate | Narrow unstable sand/shingle beach, cliff or dunes, salients | |
| 4E | Exposed | Wide unstable sand/shingle beach, cliff or dunes, salients | |
| 5P | 85o – 90o | Protected | Marshy |
| 5M | Moderate | Sandy beach, accumulative land forms, spits | |
| 5E | Exposed | Sandy beach, accumulative land forms, spits |
Special Coastal Form Elements
The following types of special coastal form elements need careful and thorough analysis in connection with shoreline management initiatives:
Delta coastlines
Deltas are formed when the supply of sediments to the coast by a river is faster than they are dispersed by waves, tides and the associated currents. Deltas can be classified according to Galloway's classification:
- Fluvial-dominated deltas are characterised by large catchment rivers discharging into relatively protected seas with minimal nearshore wave energy and a relatively small tidal range. (Example: Mississippi)
- Wave-dominated deltas are characterised by relatively high exposure by waves and/or swell, so that the wave generated transport is larger than the transports generated by river discharge and tidal exchange. (Examples: Kelantan and San Francisco)
- Tide-dominated deltas are characterised by tidal environments, where the transport of material by the tidal exchange dominates over the transport generated by waves and river discharge. (Example: Ganges-Brahmaputra)
There are many intermediate types in between the three main types. The coastal type within a delta often changes depending on its proximity to the river mouth, for which reason the coastal classification above refers to specific sections of the delta. The stability of delta coastlines is highly dependent on the supply of material from the river. Regulation work or sand mining in the rivers will often cause a deficit in the supply of material to the coastline, for which reason delta coastlines are often exposed to severe erosion. This means the development along delta coastlines shall only be performed after careful investigations of the stability of the coastline. Such investigations shall also cover all activities in the associated river, such as dam and reservoir construction, irrigation schemes, sand mining and river mouth improvement works, as all such works tend to decrease the supply of sand to the coast. To this comes the normal influence of coastal works on the stability on adjacent coastlines.
Sand spits
Sand spits of types 1M through 2E. These types of coastal features are very morphologically active formations and may be exposed to breaches when exposed to extreme wave and storm surge conditions. Consequently, development should be avoided at such locations and at adjacent stretches.
Barrier Islands
Mechanisms and conditions for barrier island development
Barrier islands are islands parallel to the shore, separated from the mainland shoreline by a lagoon. Barrier formations normally occur along coasts, where the slope of the original shoreface is flatter than the slope corresponding to the equilibrium profile, which corresponds to the actual conditions at the site. Under these conditions the waves on the shoreface will primarily transport sand towards the shore in an attempt to build up the equilibrium profile. Simultaneously, the waves lose their energy when travelling over the gentle shoreface, which means that there is not enough energy to transport the sand all the way to the shoreline. A consequence of these mechanisms is the deposition of sand some distance from the shoreline, which eventually will develop into a barrier island.
The mechanisms described above consider only cross-shore transport processes, but if sand is also brought into the area by longshore transport, this will add to the barrier formation processes. Under such conditions the island formation process can be a mixture of sand spit and barrier island processes. Barrier and sand spit formations of this kind are normally formed under type 1 or 2 conditions.
Several combinations of geological and long-term variations in water level can provide conditions for the formation of barrier islands. These conditions are discussed further in the following, see Fig. 3., which discusses a typical scenario for barrier formation on an very flat alluvial plain, which was transgressed following the last glaciation period. Following this early transgression, the water-level fell again, which led to new regression conditions.
Barrier formation
The initial barrier island, see Fig. 3. above, was formed at the maximum transgression whereas barriers 2, 3 and 4 were formed during the following regression period. At present the sea level has stabilised in the position shown, and the present barrier island formed through phases A to E. It can be seen that the initial phases of a barrier island are unstable, but the island tends to stabilise itself by moving landwards and by increasing in height. In stage, E the equilibrium profile has been reached and the barrier is more or less stable under the assumption that the sea level is constant. If the sea level rises, e.g. due to the Greenhouse effect, the barrier will tend to destabilise and to move further landwards.
Barrier formation E is characterised by the following morphological form elements. It has a normal shore with beach berms and dunes behind the coastline. The dune growth tends to stabilise the barrier formation. However, at extreme tidal wave situations, the barrier beach and dune row may be breached causing sand to be overwashed forming over-wash fans, see Fig. 3. (Form a) and the image Overwash fans at Skalligen. This situation will cause the transfer of sand from the front of the barrier to the rear of the barrier. It should be noted that the sand is not lost from the barrier, some of the sand in the breach section is just moved to the back of the barrier. This means that the sand volume in the barrier is maintained and, following the storm, the breached section will rebuild itself by natural processes. However, the breaching will result in a general recession of the shoreline before a new equilibrium situation is reached.
There are also other mechanisms, which contribute to the destabilisation of the barrier islands. Considering situation E, where there is a series of barrier islands separated by tidal inlets, the shifting tidal currents will tend to form flood and ebb shoals, see form b in barrier formation figure. This requires considerable volumes of sand, which are taken from the barrier form complex. If there is a littoral transport along the islands, this may supply sand from adjacent beaches and/or it may transfer sand to the tidal inlets, where it is lost in the shoals.
The lengths of the islands, or the distances between the tidal inlets, are dependent on the tidal volume. If the tidal volume is large, the distance between the inlets will be relatively short and vice versa. The dimensions of the tidal inlets are also dependent on the tidal volume as well as the littoral transport conditions.
Effect of development
Many tidal inlets are regulated by jetties and dredging. This will normally lead to loss of sand from the barrier system, either in an upstream accumulation, by offshore loss or by dumping of dredged material offshore or elsewhere outside the system. The regulation of tidal inlets is actually one of the most common causes of the destabilisation of barrier islands. Well-known examples of this are the regulated tidal inlets along the American East Coast and the West Coast of Florida.
It is evident from the above description that barrier islands are active systems, which are natural stable form elements, although only if slow backward movement is allowed. When a barrier island has been occupied by housing and other development, this backward movement can no longer be accepted. This situation, combined with the above-mentioned inlet problems, is the cause of many severe erosion problems along barrier island shorelines. If it is not already too late, development should be avoided close to the coastline on barrier islands.
Coastlines close to river mouths and to tidal inlets. Such locations are very morphologically active formations as they are formed by the interaction between several hydrodynamic processes such as littoral processes, river discharge and tidal currents. Furthermore, river mouths and tidal inlets are often used for navigation and they are natural locations for towns and cities. There will often be a port of some kind, and the channels are often regulated by dredging and inlet structures. Furthermore, there can be stratification and associated additional sedimentation. Consequently, these types of coastal areas are often suffering from a series of interrelated problems, such as coastal erosion, flooding, sedimentation and navigation constraints. New coastal development should not be allowed close to natural river mouths and tidal inlets; the solution of specific problems requires thorough investigation.
Accumulating sand spits. This type of coastal feature most often occurs on coasts of types 5M and 5E, and maybe also in connection with 4M and 4E. This type of feature is often formed in areas with a decreasing littoral drift rate, which means that the supply of littoral material to a section is greater than the transport capacity out of the section. However, for very oblique wave attack, the coastline development is often unstable and shows a tendency to form coast-parallel features at some distance from the coastline. A characteristic of this separated spit formation is that the coastline downstream of the spit has no supply of sediments, and consequently will be exposed to erosion. The construction of coastal structures, which protrude into the shoreface area, may initiate the development of a coast-parallel shoal, for which reason such structures should be avoided at type 4 and 5 coastlines. As it is very difficult to predict how such a coastline will change, development close to such a coastline should be avoided.
Further reading
Mangor, K., Drønen, N. K., Kaergaard, K.H. and Kristensen, N.E. 2017. Shoreline management guidelines. DHI https://www.dhigroup.com/marine-water/ebook-shoreline-management-guidelines
Please note that others may also have edited the contents of this article.
|
